Field of Invention
This invention relates generally to electromagnetic flowmeters, and more particularly to a flowmeter which operates at a relatively low average power level, yet has an exceptionally high signal-to-noise ratio.
In an electromagnetic flowmeter, the fluid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically-opposed electrodes, a magnetic field perpendicular to the longitudinal axis of the tube being established by an electromagnet. When the flowing liquid intersects the field, a voltage is induced therein which is transferred to the electrodes. The voltage, which is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed to actuate a recorder to indicator.
The magnetic field may be either direct or alternating in nature, for in either event the amplitude of voltage induced in the liquid passing through the field will be a function of its flow rate. However, when operating with direct magnetic flux, the D-C signal current flowing through the liquid acts to polarize the electrodes, the magnitude of polarization being proportional to the time integral of the polarization current. With alternating magnetic flux operation, polarization is rendered negligible, for the resultant signal current is alternating and therefore its integral does not build up with time.
Though A-C operation is clearly advantageous in that polarization is obviated and the A-C flow induced signal may be more easily amplified, it has distinct drawbacks. The use of an alternating flux introduces spurious voltages that are unrelated to flow rate and, if untreated, give rise to inaccurate indications.
The adverse effects encountered in A-C operation of electromagnetic flowmeters can be attributed to the rate of change of the flux field (d.phi.)/dt, serving to induce unwanted signals in the pick-up loop constituted by the electrodes and the liquid bridging the electrodes. If, therefore, the rate of change of the flux field could be reduced to zero value, then the magnitude of quadrature and of its in-phase component would become non-existent. Zero drift effects would disappear.
When the magnetic flux field is a steady state field, as, for example, with continuous d-c operation, the ideal condition d.phi./dt=0 is satisfied. But, as previously noted, d-c operation to create a steady state field is not acceptable, for galvanic potentials are produced and polarization is encountered.
In U.S. Pat. No. 4,296,636 to Mannherz and in U.S. Pat. No. 3,783,687 to Mannherz et al., whose entire disclosures are incorporated herein by reference, there are disclosed electromagnetic flowmeters in which the excitation current for the electromagnetic coil is a low-frequency wave serving to produce a periodically-reversed steady state flux field, whereby unwanted in-phase and quadrature components are minimized without giving rise to polarization and galvanic effects. This low frequency wave may be derived by means of a presettable scaler coupled to the standard a-c power line (60 Hz) and is at a frequency in the order of 17/8, 33/4, 71/2 or 15 Hz.
A flowmeter arrangement which in many respects is similar to that disclosed in the Mannherz et al. patent is described in the Schmoock U.S. Pat. No. 4,370,892, whose entire disclosure is incorporated herein by reference.
A major drawback of the conventional drive system for an electromagnetic flowmeter is that its power requirements are substantial. This precludes the use of battery operation. Also, as will later be explained, the signal-to-noise ratio of a conventional flowmeter is such that when the prevailing noise level is high, it is difficult to detect the signal that reflects flow rate.
When the fluid being metered takes the form of a coarse slurry containing solid particles such as sand, fly ash or salt which impinge on the surface of the electrodes as the slurry passes through the meter tube, it has been found that a substantial noise component is generated with a conventional flowmeter. This makes signal detection more difficult and in some instances impossible. The meter electrodes in combination with the fluid acting as an electrolyte define a galvanic cell, and when the solids in the slurry strike the electrodes and alter their interface to the fluid, this action brings about a rapid change in galvanic voltage, thereby generating noise. A second source of noise arises when the flowmeter is run partially full. In the case of electrodes which make direct contact with the fluid, the resultant sloshing of the fluid on the surface of the electrodes produces excessive galvanic noise.
Noise is any voltage that does not convey measurement information. Under the most favorable circumstances where noise has been minimized by filtering or other expedients, there are still certain sources of noise present resulting from the granular nature of matter and energy. While noise fluctuations may be small compared with the total energy transfer involved in most measurements, the existence of a noise background limits the ultimate sensitivity to which a measurement can be carried.